Differentiating cells undergo a number of changes which affect the cellular response to various compounds or to new media conditions. Whereas exposure of Bacillus subtilis to fusidic acid during exponential growth, cells become resistant during development. This resistance appears when prespore septa and resistant ribosomes are found. Later, when the forespore has been completely engulfed by a double membrane, sporulation is committed to continue even if cells are exposed to a fresh, rich medium. Simultaneously, the mother cells lose their ability to grow. Apparently, forespores secrete a compound that causes the deterioration of the mother cells. Butyric acid causes drastic changes in Hela and other cells. Some of which mimic the effect of hormones that seem to stiffen the lysosomal membrane. Under the light and electron microscope, stains for acid phosphatase and glucuronidase indicate that these enzymes are more concentrated in lysosomal particles when cells are grown with butyrate. BIBLIOGRAPHIC REFERENCES: Fortnagel, R., and Freese, E.B.: Morphological stages of Bacillus subtilis sporulation and resistance to fusidic acid. J. Gen. Microbiol. 1977, (in press). Freese, E.B., and Freese, E.: The influence of the developing bacterial spore on the mother cell. Dev. Biol., 1977, (in press).